CTENODONTIDiB. 
253 
( t .) determine the anterior extremity of the specimen, and 
suggest its probable reference to C. cristatus. Nearly all 
the bones are considerably fractured on the external 
surface, and some are shown in little more than impres¬ 
sions ; but the approximate outlines of most of the 
elements of the cranial shield appear to be distinguishable. 
Hindermost is a large median plate (0) elongated antero- 
posteriorly, and having the anterior margin produced 
mesially into a short triangular projection between the 
posterior extremities of the narrow pair of elements (I) 
immediately in front. The last-named bones are only in 
contact in the middle line of the skull for about half their 
extent in advance of the process of the hinder mesial 
element, being soon separated by another, though com¬ 
paratively small and narrow, azygous bone (0) ; and this 
likewise extends between the hinder ends of a second 
larger pair (I), which would be originally in direct contact 
with the anterior ends of the first pair. This median 
series of bones is immediately flanked by four pairs of 
large broad bones, of which the first (II) and half of the 
second adjoin the hindermost element, while the third is in 
contact with both pairs of series I., and the fourth probably 
with the anterior inner pair alone. Still more externally 
there occurs another series of broad alternating bones on 
either side (III), of which only few fragments are preserved. 
On the whole, it will be noticed that there is a remark¬ 
able resemblance to the arrangement of the plates in the 
cranial shield of Dvpterus (fig. 37), as already recognized 
by Hancock and Atthey 1 ; the only striking difference 
being the apparent subdivision of some of the elements in 
the Devonian genus. Moreover, the median series of 
bones is arranged exactly as in Acipenser and Polyodon 2 . 
Purchased , 1885. 
388-57. Crushed remains of the head ; Airdrie, Lanarkshire. The 
dental plates are much abraded and imperfectly exposed ; 
a few of the posterior cranial roof-bones are distinguish¬ 
able ; and there are also portions of the palate, though 
broken almost beyond recognition. Purchased , 1864 
45857. Operculum ; Newsham. Purchased , 1874. 
1 Nat. Hist. Trans. Northumb. & Durham, vol. iv. p. 401. 
2 T. W. Bridge, Phil. Trans. 1878, p. 684, pi. Iv. 
